Myrmecochorous plants in Mediterranean region and their dispersal by ants

I. Li Vigni, B. Paternostro, V. Giusquiano

Banca di Germoplasma del Mediterraneo ONLUS - Via P. Floridia n° 2 - 90129 Palermo (I)

Internet http:// www.bancadigermoplasma.it

X OPTIMA Meeting Palermo, 13-19 September 2001

INTRODUCTION

Myrmecochory, specifically, is the transport of seeds by ants, with the ants eating the only edible part of the seed: elaiosome. These ants do not kill the seeds, which still can germinate far from the place where they originated.

Myrmecochory was first studied in depth by Sernander, a botanist at the University of Uppsala in Sweden. In 1906 he published a distinguished review on European myrmecochorous shrubs. Nowadays we know of more than 4,000 species of myrmecochorous Angiospermae belonging to more than 70 families, who’s diasporas are dispersed by ants (Li Vigni and Melati 1999).

Seed dispersal by ants is important to many species of plants, especially in eastern North America and Australia (Beattie 1985, Gunther and Lanza 1989). Dispersion of seeds by ants happens in temperate and tropical areas but seems prevalent in dry areas such as Australia, the Sahel zone and South Africa; it is also often carried on by harvester ants, even though normally they kill the seeds. There is a general increase in ant-dispersed plant species from the poles to the equator that parallels the increasing diversity of ants across the same gradient. However, there may be a peak of abundance at the warm-temperate and Mediterranean midlatitudes (Beattie 1983).

Myrmecochory is an almost worldwide phenomenon:

1) Ants Formicinae and seeds of Angiospermophyta (Europe).

2) Ants Aphaenogaster rudis and seeds of Sanguinaria canadensis (Europe and North America).

3) Formica sp. and seed (North America).

4) Ant Pseudomyrmex sp. and seed of Acacia sp. (Central America).

5) Ant Pachycondyla goeldii and seed of Anthurium sp. (South America).

6) Ants Ectatoma ruidum and seeds of Piper culebranum (South America).

7) Ants of seeds (Australia).

8) Ants and seed of “fynbos” (South Africa).

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MATERIAL AND METHODS

This study examines plants and insects found in the Mediterranean region (Sicily), observed in their natural habitat in three different locations: the Ficuzza woods, the Madonie mountains and the coastline of Addaura. In each plant community, we investigated the relative effectiveness of myrmecochory, by experimentally offering elaiosome-bearing seeds of Viola odorata, Portulaca oleracea, Chelidonium majus and Borago officinalis, and seeds of the same species deprived of their elaiosome.

RESULTS AND DISCUSSION

The ant species known to disperse seeds and fruits all belong to one of four subfamilies of the Formicidae: the Ponerinae, the Myrmicinae, the Dolichoderinae and the Formicinae. Species such Acantholepis nigra, Camponotus barbaricus, Camponotus ligniperda, Camponotus policornis ssp. siculus, Formica lugubris, Formica sanguinea, Lasius alienatus, Lasius niger, Lasius flavus and Plagiolepis pygmaea (Formicinae) are found throughout the Mediterranean region.

Elaiosomes are found both in seed (e.g. in Ricinus communis, Viola odorata, Portulaca oleracea, Corydalis cava, Poligala vulgaris, Lunula campestris and Chelidonium majus) and in fruit (e.g. Borago officinalis and Lamium album) and appear in plants in no way systematically, so that in these cases we can talk about converging evolution (Li Vigni 1986).

Some ants in temperates areas, such as Formica rufa and related species carry seeds to use in building their nest. The most favoured seeds are ones with an elaiosome or oily body or ones, which exude oil, too cover the test of the seeds. The number of seeds carried by one ant alone con is considerable (almost 40,000 for only one worker of Formica rufa in 80 days). The distance travelled can be as far as 70 m (Jolivet 1998).

We suggest that myrmecochory is underrepresented because of biogeographical features (rarity of the medio-Europaean flora) and lack of strong selective pressure.

LITERATURE

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